I would argue that the only true test of mental illness would be "Have you received Jesus as your personal Savior?"
If the answer is 'yes' they are certifiably sane.
If the answer is 'no' they are borderline. Present Christ to them and then reask the question.
If the answer is otherwise negative and emotional they are certifiably insane.
The reason I place sanity under the condition of what a person has done with Christ is that those who reject Christ are masochistic.